Keyless vehicle access and starting systems, for example, the passive start entry (PASE) system, are automatic systems for unlocking a vehicle without actively using a car key and for starting it by merely actuating the start button. This is enabled by an electronic key with chip, which the vehicle driver carries with him. A coded response signal is periodically emitted by the vehicle via at least one antenna located on the vehicle on an LF frequency (LF stands for “low frequency” having frequencies between 20 kHz and 200 kHz, for example). The system then goes into a receiving mode in the UHF range (UHF stands for “ultra high frequency” having frequencies in the three-digit megahertz range, for example) and waits for confirmation. If a key equipped with a transponder is in range, it receives the LF signal, decodes it, and emits it again with a new coding as the UHF signal. The UHF signal is decoded in the vehicle. Since the vehicle knows both coding tables, it can compare the actual original emission to the signal just received and permit access if they correspond. If there is no correct response within a defined time, nothing happens and the system switches back to standby. The engine start procedure essentially corresponds to that of the access control, except that the engine start button is actuated here.
An inductive antenna, which is embodied, for example, as a ferrite core provided with a winding (also referred to as a magnet antenna or ferrite antenna) is predominantly used as the antenna for emitting the LF signal. The inductor coil of the inductive antenna is frequently driven in this case together with a capacitor in a resonant circuit. The power consumption of such a resonant circuit is typically kept low by the highest possible quality and exact frequency tuning, to keep the overall power consumption of the access and starting system as low as possible. A low power consumption is already desirable solely because, for example, otherwise the vehicle battery would be rapidly discharged during a longer shutdown time of the vehicle. A high quality restricts the transmission data rate, however, and exact tuning at high quality requires some expenditure. Common arrangements therefore usually represent an unsatisfactory compromise between data rate, expenditure, and power consumption.